Download Phys 214. Planets and Life

Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: [email protected]
(Please use PHYS214 in e-mail subject)
Lecture 9.
The nebular theory + Movie
(Page 74-80)
January 25
Contents
Textbook: Pages 74-80
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The origin of our solar system
Nebular theory
Planetary nebulae
Should habitable worlds be common?
• Movie
• Acknowledgments: NASA, ESA, Hubble
The origin of our Solar System- nebular theory
The origin of our Solar system might give us some insight into finding
habitable worlds in other star systems.
Nebular theory – our solar system was born from the gravitational
collapse of an interstellar cloud, or nebula, of gas and dust.
Carina nebula
Nebular theory
Nebular theory
The formation of the
solar system
according to the
nebular theory has
four steps:
1. Contraction
2. Condensation
3. Accretion
4. Clearing
Nebular theory
1.
Contraction
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The solar nebula began as a large,
diffuse cloud, roughly spherical in
shape.
The initial cause of collapse is
unknown, perhaps a nearby supernova.
Similar clouds exist today and they
show they can collapse and give birth
to stars.
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Once the gravitational collapse
begins
• the solar nebula heats up
• spin faster
• flatten into a disk
• shrinks in size
Nebular theory
The solar nebula heats up <- law of energy conservation.
Large gravitational potential energy -> kinetic energy & heat as they fall inward and
collide. The cloud becomes hotter near the center, where the star forms.
Spin faster <- conservation of angular momentum.
The total amount of circling motion of an object must be conserved. A shrinking cloud
spins faster as it contracts.
Flatten into a disk <- consequence of the spin.
When the particles collide, they tend to add to each other’s motion when they move in
the same direction. However, they cancel each other’s motion in other directions.
Nebular theory
The overall composition of the galaxy and Sun
implies that:
The composition of the solar nebula was:
- 98% hydrogen and helium, &
- 2% other elements (essential for planet
formation) - metal, rock, and hydrogen
compounds (water, methane, amonia).
The process of planet formation in the early solar
system is best described by condensation.
Nebular theory
2. Condensation
The materials present in the solar system with
the highest condensation temperatures
were metals.
Because the temperatures were high in the
inner solar system, only materials with
high condensation temperatures could
become solid (metals and rock)
Nebular theory
2. Condensation
Metal and rock compounds (with high condensation T) could condense within about the
present location of the asteroid belt.
Farther out, where temperatures were much lower, in addition to metal and rock, hydrogen
compounds could condense to make ice.
Nebular theory
3. Accretion and terrestrial planet formation
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Solid particles grew larger = accretion.
Particles orbit the forming Sun with orderly circular paths, each particle moving at
about the same speed as neighbouring particles.
Gentle collisions - due to electrostatic forces and not to gravity.
Particles grew larger in mass -> gravity sticks them together into boulders planetesimals (protoplanets or pieces of planets).
Nebular theory
3. Accretion and terrestrial planet formation
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Planetesimals grew to hundreds of km -a few million years (only 1/1000 the
present age of the solar system).
Dozens or even hundreds of planetesimals orbiting the Sun between the present day
orbits or Mercury and Mars.
Continued accreting - sometimes colliding violently.
Computer simulations reproduce the collapse of clouds in spinning disks and the
first stages of of accretion; cannot predict the results of late stages of accretion,
especially the balance between shattering collisions and planetesimal growth.
Illustration Credit: T. Pyle (SCSC),JPL-Caltech, NASA
Nebular theory
3. Accretion and terrestrial planet formation
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In the inner solar system, at least 4 objects grew to planetary size, becoming Mercury,
Venus, Earth, and Mars (4.900 –12,000 km).
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At least a few other Moon (3,400 km) -to Mars (6,800 km) size objects might have been
present in early times, but eventually smashed into one of the four planets that survived .
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Moon formed when a Mars-size object collided violently with the young Earth.
Nebular theory
3. Accretion and jovian planet formation (scientific debate).
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The planetesimals in the outer solar system contained a larger amount of ice in
addition to metal and rock.
rock.
All solids objects that reside in the outer solar system today, such as comets,
Kuiper belt objects, moons of jovian planets,
planets, all show an ice-rich composition.
The Jovian planets most likely formed from planetesimals of rock and ice attracting
hydrogen and helium gas from the solar nebula
Nebular theory
3. Accretion and moons of jovian planet formation
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Similar process to the one that made the disk of the solar nebula (heating, spinning
flattening).
Each jovian planet - surrounded by its own disk of gas, spinning in the same
direction that the planet rotates.
Moons - accreted from ice-rick planetesimals within the disks, closed to the
equatorial plane of the planet.
This model explains why jovian planets have many moons.
Nebular theory
3. Accretion and jovian planet
formation
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The model of accretion
followed by gas capture
explains the observed features
of the jovian planets well.
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A competing model suggests
that disturbances within the
disk of the solar nebula led to
clumps of gas to collapse and
form jovian planets without
the need of forming icy
planetesimals first.
Nebular theory
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4. Clearing the disk
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As the planets formed, the Sun also
formed and accreted the remaining
gas.
Young Sun had a strong solar wind,
blowing off particles from its surface
out into space.
The wind swept away the remaining
gas into the interstellar space, ending
the era of planet formation.
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Nebular theory – clearing the disk
• Once nuclear ignition is achieved the star releases a massive wind
-sweep out the remaining gas (T Tauri phase)
• The remaining planetesimals close to the Sun will almost all
impact with planets in this region
–creation of the Moon
–About 20,000 of these objects left between Mars & Jupiter
–The rate of impacts was clearly much higher in the past than
it is now
•Planetesimals farther out (mostly icy)
interact with the Jovian planets and
can be thrown out of the solar system!
(comets)
Disk of dust
Three trillion mile-long jet from a star
hiding in dust
Explaining the worlds – planets rotation
Nebular theory predictions:
Planets rotate in the same direction as they
orbit the Sun and in the same plane.
Sun rotates in the same direction (born at
the centre of the spinning cloud).
The general cloud rotation explains why
most planets rotate in the same direction
and most of the large moons orbit their
planets in the same direction.
Explaining the worlds – planets rotation
Nebular theory:
The condensation theory does not explain
the rotation rate of the planets.
Explaining the worlds – almost circular orbits
Planets have nearly circular orbits,
because particles with more
elliptical orbits would have suffered
more collisions.
Most planetesimals ended up in one of
the eight major planets; many
planetesimals shattered into pieces.
Asteroids are the remaining
planetesimals of the inner solar
system. Most reside in the asteroid
belt, others in Kuiper belt.
Explaining the worlds
Oort cloud comets - more difficult to explain.
originated as comets orbiting among jovian
planets.
When they passed near a jovian planet, they
were flung out to a great distance by the
planet gravity (similar to the way the scientists
use Jupiter’s gravity to accelerate spacecraft to
planets beyond).
Explaining the worlds - Exceptions
Uranus is tilted at 98° to the plane of the solar system
• Possibly an off-centre impact, or the fact that the solar
nebular is less dense in the outer parts allowing a higher
probability of being at an angle
Pluto and Mercury lie at 7 and 17 degrees relative to the plane
of the solar system
• Mercury probably suffered an impact during its formation
(it is small and easy to perturb)
• Pluto seems to be a left over planetesimal so probably had
many encounters to knock it into a strange position
Explaining the worlds - Exceptions
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Moons with strange orbits - Triton which
orbits opposite to Neptune’s rotation
Probably a captured planetesimal
• Earth’s moon orbits in the plane of the
solar system, not in the plane of the
Earth’s equator
Impact event occurred in the plane of the
solar system
Should habitable worlds be common?
Formation of the spinning disk consequence of physical laws that
operate everywhere
-> most stars surrounded by spinning disks in
which planets may form.
Observations support this idea – many young
stars have such disks!
Hubble telescope photo - flattened spinning
disk around star AU Microscopii (edge on).
Light reflected off dust around the young star
when the star light is blocked.
Hubble Space Telescope near-infrared picture
of a disk around the star HD 141569, located
about 320 light-years away in the constellation
Libra.
Planetary nebulae
Hubble Space Telescope images of four protoplanetary disks around young stars in the Orion nebula, located 1500 lightyears away. Gas and dust disks can be seen in visible light.
Planetary nebulae
Protoplanetary disks in the Orion Nebula as seen by Hubble Edge
on.
Artist impression - HD 98800 system - two pairs of double stars,
with one pair surrounded by a disk of dust. Recent data from
the Earth-trailing Spitzer Space Telescope in infrared light,
indicate that the dust disk has gaps consistent with being
cleared by planets orbiting in the disk. If so, one planet appears
to be orbiting at a distance similar to Mars of our own Solar
System.
Should habitable worlds be common?
We expect to find many planetary systems with
terrestrial and jovian planets laid out the same.
However, most of the extrasolar planetary systems
discovered to date are quite different than our
own solar system:
- jovian planets found close to their parent stars.
- planets orbit closer to their star (closer than
Mercury’s orbit).
Movie. The discovery of most Earth-like planet.
The first image of an extrasolar planet. The planet roughly five
times the mass of Jupiter is orbiting a brown dwarf.
Artificial-colour Hubble Near Infrared Camera and MultiObject Spectrometer (NICMOS) infrared-light view of the
brown dwarf star 2M1207 and giant planet companion
candidate - about five times the mass of Jupiter, is the magenta
coloured spot at lower right. The brown dwarf’s location is
within the circle at image centre. The glare of the dwarf, which
is 700 times brighter than the planet candidate has been greatly
reduced through image processing
Should habitable worlds be common?
We cannot say with certainty whether solar
systems like ours should be rare or
common.
However, rare in Milky Way means large
actually. If only 1 in 1 million star has a
system like ours, this is 100,000 systems!
Therefore, it is almost inevitable that our
galaxy contains many worlds that have
liquid water and would be suitable for life.
Movie
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Hubble 15 years of discovery
Chapter 3. Planetary tales (9 minutes)
Next lecture
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Chapter 4. The habitability of Earth
Geology